1. Field of the Invention
The present invention relates to an absolute position length measurement type encoder that includes a scale in which an ABS pattern based on a pseudorandom code is provided, a light-receiving element for receiving bright and dark patterns formed by the scale, and a signal processing circuit for processing signals in compliance with output of the light-receiving element and measuring an absolute position of the scale with respect to the light-receiving element, and in particular to an absolute position length measurement type encoder capable of measuring a moving distance at high accuracy by arithmetic operations for correlation without being influenced by an array interval of ABS light-receiving element array of the light-receiving element.
2. Description of the Related Art
Accurate position control and measurement are indispensable in measuring instruments and apparatuses. Therefore, an absolute position length measurement type encoder capable of executing absolute position measurement over some length has been used. In particular, where high accuracy is required, a photoelectric type encoder has been used.
Generally, an absolute position length measurement type photoelectric encoder has an absolute pattern (hereinafter called an ABS pattern) to roughly measure a moving distance and an incremental pattern (hereinafter called an INC pattern) to measure a moving distance at high resolution power by interpolating the interval of a roughly obtained moving distance on a scale. The light-receiving element includes ABS light-receiving element array for ABS pattern and INC light-receiving element array for INC pattern.
Since bright and dark patterns formed by ABS pattern and INC pattern, which are formed on the scale, change on the light-receiving element, which receives the bright and dark patterns, the moving distance can be measured at high accuracy by processing the change in a signal processing circuit. The ABS pattern is prepared based on a pseudorandom code, and patterns to be used can be made into one, wherein the absolute position length measurement type encoder itself can be downsized. Also, since highly accurate measurement is enabled for a moving distance in measurement of the absolute position, a method for arithmetic operation for correlation has been used (For example, Japanese Published Unexamined Patent Application No. 2002-230560 (hereinafter called Patent Document 1), and Japanese Published Unexamined Patent Application No. 2006-226987 (hereinafter called Patent Document 2)).
However, digital signals used for arithmetic operations for correlation are discrete data obtained per array interval of the ABS light-receiving element array. Therefore, when carrying out an arithmetic operation for correlation using the discrete data, it was assumed that an integral multiple of the array interval of the ABS light-receiving element array is equal to the minimum line width of the ABS pattern based on the pseudorandom code being an object of the arithmetic operation for correlation (Patent Documents 1 and 2). However, there is a restriction in the array interval of the ABS light-receiving element array, freedom for selection of the light-receiving elements was necessarily reduced. Also, the degree of freedom in design for the minimum line width of the ABS pattern could not be sufficiently secured. As a result, there is a problem in that adverse influences are given to the size and performance of the absolute position length measurement type encoder and to the production cost thereof.